Piezoelectric actuators are used in a variety of applications where it is desired to obtain electronic control over small physical displacements. One such application is the piezoelectric actuated deformable mirror comprising, for example, a peripherally clamped reflecting membrane having one or more regions coupled to respective piezoelectric actuators. The displacement of the piezoelectric actuators and, hence, the geometry of the reflective membrane can be controlled by the voltage applied to the actuators.
One difficulty that has arisen in the use of piezoelectric actuators is the problem of hysteresis effects. High gain piezoelectric ceramics typically exhibit a large ferroelectric effect which manifests itself as hysteresis in the displacement versus voltage response as the applied voltage cycles between two values. As a consequence of these hysteresis effects, the displacement response curve tends to form a loop rather than a straight line with the consequence that displacement is not precisely linear with voltage and is not a single valued function of voltage.
These hysteresis effects pose severe problems in applications where highly accurate dimensional control is required. The variation in displacement at the mid-loop voltage value can be as high as twenty percent for typical high gain actuators; and because displacement is not a single-valued function of voltage, actuator displacement control cannot be obtained by simply controlling the actuator voltage.
Prior attempts to overcome this problem have typically resulted in bulky apparatus which additionally is either highly expensive or unduly slow in frequency response. One such approach involves adding position transducers to monitor displacement in a feedback loop. These position transducers, however, are bulky and expensive.
Another involves foregoing the use of high gain piezoelectric materials and using, in their place, lower gain piezoelectric materials chosen for low hysteresis and arranged in long stacks to accomplish the desired displacement. The difficulty with this approach is such actuators are typically bulkier and have a lower frequency response than those made from high gain material.
A third approach is disclosed in U.S. Pat. No. 3,916,226 issued to D. B. Knoll. Knoll regulates the piezoelectric actuator by utilizing a specifically formed excitation waveform. The actuator is supplied with a constant current over a predetermined first time interval establishing a charge of a first polarity which is then completely withdrawn during a second time interval to return the transducer to its initial value. The Knoll approach, however, is unduly complex.
Accordingly, it is desirable to provide an improved piezoelectric actuator design for reducing the effects of hysteresis.